Field
The present invention relates to the field of hydrocarbon recovery procedures. More specifically, the present invention relates to the isolation of a subsurface formation using an improved bridge plug arrangement for the purpose of injecting fluids.
Discussion of Technology
In the drilling of oil and gas wells, a wellbore is formed using a drill bit that is urged downwardly at a lower end of a drill string. After drilling to a predetermined depth, the drill string and bit are removed and the wellbore is lined with a string of casing. An annular area is thus formed between the string of casing and the formation. A cementing operation is typically conducted in order to fill or “squeeze” the annular area with cement. The combination of cement and casing strengthens the wellbore and facilitates the isolation of certain areas of the formation behind the casing for the production of hydrocarbons.
It is common to place several strings of casing having progressively smaller outer diameters into the wellbore. Thus, the process of drilling and then cementing progressively smaller strings of casing is repeated several times until the well has reached total depth. The final string of casing, referred to as a production casing, is cemented into place. In some instances, the final string of casing is a liner, that is, a string of casing that is not tied back to the surface.
As part of the completion process, the production casing or liner is perforated at a desired level (or levels). Additionally or alternatively, a sand screen may be employed depending on the conditions of the well and the formation. Either option provides fluid communication between the wellbore and a selected zone in a formation. In addition, production equipment such as tubing, packers and pumps may be installed within the wellbore. A wellhead is installed at the surface along with fluid gathering and processing equipment. Production operations may then commence.
Before beginning production, it is sometimes desirable for the drilling company to “stimulate” the formation by injecting an acid solution through the perforations. This is particularly true when the formation comprises carbonate rock. The drilling company typically injects a concentrated formic acid or other acidic composition into the wellbore, and directs the fluid into the zone of interest. This is known as acidizing. The acid helps to dissolve carbonate material, thereby opening up porous channels through which hydrocarbon fluids may flow into the wellbore. In addition, the acid helps to dissolve drilling mud that may have invaded the formation. Thus, acidizing may increase the effective diameter of the wellbore.
After a period of time, production from the zone of interest may begin to taper off. When this occurs, it is sometimes possible to restore the production rate of hydrocarbons by perforating the casing at a new zone of interest at a more shallow depth within the formation. The new zone of interest (or new formation as the case may be), may also undergo acidizing so as to increase permeability of the rock.
To direct the acidizing solution into the new zone of interest, it is desirable to temporarily seal off the wellbore below the new zone of interest to prevent the acidizing solution from preferentially invading the original formation therebelow. To do this, the operator will employ a fluid diversion technique. Two general categories of fluid diversion have been developed to help ensure that the acid reaches the desired rock matrix—mechanical and chemical. Mechanical diversion involves the use of a physical or mechanical diverter that is placed within the wellbore. Chemical diversion, on the other hand, involves the injection of a fluid or particles into the formation itself.
Referring first to chemical diverters, chemical diverters include foams, particulates, gels, and viscosified fluids. Foam commonly comprises a dispersion of gas and liquid wherein a gas is in a non-continuous phase and liquid is in a continuous phase. Where acid is used as the liquid phase, the mixture is referred to as a foamed acid. In either event, as the foam mixture is pumped downhole and into the porous medium that comprises the original, more permeable formation, additional foam is generated. The foam initially builds up in the areas of high permeability until it provides enough resistance to force the acid into the new zone of interest having a lower permeability. The acid is then able to open up pores and channels in the new formation.
Particulate diverters consist of fine particles. Examples of known particulate diverters are cellophane flakes, oyster shells, crushed limestone, gilsonite, oil-soluble naphthalenes, and even chicken feed. Within the last several years, solid organic acids such as lactic acid flakes have been used. As the particles are injected, they form a low permeability filter-cake on the face of wormholes and other areas of high permeability in the original formation. This then forces acid treatment to enter the new zone(s) of interest. After the acidizing treatment is completed, the particulates hydrolyze in the presence of water and are converted into acid.
Viscous diverters are highly viscous materials, sometimes referred to as gels. Gels use either a polymer or a viscoelastic surfactant (VES) to provide the needed viscosity. Polymer-based diverters crosslink to form a viscous network upon reaction with the formation. The crosslink breaks upon continued reaction and/or with an internal breaker. VES-based diverters increase viscosity by a change in micelle structure upon reaction with the formation. As the high-viscosity material is injected into the formation, it fills existing wormholes. This allows acid to be injected into areas of lower permeability higher in the wellbore. The viscosity of the gel breaks upon exposure to hydrocarbons (on flowback) or upon contact with a solvent.
Referring now to mechanical diverters, various types of mechanical diverters have been employed. These generally include ball sealers, plugs, and straddle packers. For example, U.S. Pat. No. 3,289,762 uses a ball that seats in a baffle to cause mechanical isolation. U.S. Pat. No. 5,398,763 uses a wireline to set and then to retrieve a baffle. The baffle isolates a portion of a formation for the injection of fluids. U.S. Pat. No. 6,491,116 provides a fracturing plug, or “frac plug.” Frac plugs are common in the industry and rely upon a ball that is either dropped from the surface to land on a seat, or that is integral to the plug itself. Frac plugs generally require a wireline for setting. Frac plugs may also be retrieved via wireline, although in some instances frac plugs have been fabricated from materials that can be drilled out. Drilling out the material adds time and expense to the stimulation operation.
The concept of destructible plugs has also been introduced to the industry. SPE Paper No. 102,994-MS teaches an internal explosive that causes a plug to fall into the rat hole. See L. Swor and A. Sonnefeld, Self-Removing Frangible Bridge Plug and Fracture Plug, Society of Petroleum Engineers Paper No. 102,994-MS (2006). The plug is set on wireline, used for fluid diversion, and destroyed using internal timed explosives that are activated at the surface. The plug will detonate at a set time downhole and there is no stopping it if other issues arise. U.S. Pat. No. 5,924,696 presents a frangible pressure seal that is used in conjunction with packers and sealing members and a shoulder-type seat. Other systems use a plug that incorporates high strength glass as part of the mechanical isolation. The plug contains an explosive element that is detonated remotely. These systems typically result in a permanent restriction in the wellbore due to the presence of the seat. They also have the complexity of running the plug and then using explosives for detonation.
U.S. Publication No. 2007/0204986 A1 discloses a tubing plug that must be preinstalled in a premium connection. Removing it requires drilling or milling for removal, similar to cast-iron bridge plugs. Milling and drilling are expensive, risky, and time consuming operations. To form a hydraulic seal, the plug relies upon a seal bore assembly. The plug relies upon a premium pin and box assembly to support and retain the plug.
While mechanical plugs can provide high confidence that formation treatment fluid is being diverted, there is a risk of incurring high costs due to mechanical and operational complexity of the plugs. Plugs may become stuck in the casing resulting in a lengthy and costly fishing operation. If unsuccessful, a drill rig may be needed to be brought on-sight to drill the plug out. Drilling out the plug is not preferred due to the time and cost associated with mobilizing a drill rig on location. In some situations, the well may have to be sidetracked or even abandoned. Mechanical plugs particularly have a history of reliability issues in large diameter wells. In this respect, it can be difficult to locate a plug suitable for a large borehole, and those that are available have a history of failures.